In screen printing processes a woven screen or fabric material is used for printing by forcing ink through the interstices of the weave or mesh. A wide range of fabrics may be used with thicknesses between 0.001 and 0.020 inch and with thread counts varying from 12 to 600 per inch. The image to be printed is permanently formed on the screen by blocking or stenciling certain of the interstices using a photo etching process or the like. To achieve high quality results with good registration and resolution, it is necessary for the screen to be maintained stable and high under tension. A frame that can stretch a screen equally along the linear length of at least one side thereof to any desired tension and can maintain the screen in that condition during printing is shown in U.S. Pat. Nos. 3,601,912 and 4,525,909.
One problem associated with screen printing of the type heretofore mentioned, particularly within the processes that require high accuracy and precision in the ultimate image, is the misalignment of the printed image. This misalignment is likely due to a number of factors and tolerances that can change during each print. The misalignments in the printed image may be created during () the formation of the original image or artwork which may not be dimensionally correct to the desired image to be reproduced, (2) the formation of a film positive art work which may vary in dimension as compared to the original art work due to temperature and humidity considerations or due to camera technique, (3) the burning of the stencil into the stretched screen which may cause dimensional changes in the ultimate image (even if the original art and the film positive image are perfect dimensionally), (4) the application of the ink onto the screen via the squeegee which stretches the screen so as to contact with the printing surface and elongates the image on the screen (although higher screen tension greatly minimizes, or substantially eliminates, this factor in contributing to the tolerance accumulation), (5) the printing process due to temperature and humidity considerations on the stencil, (6) the printing process due to the variation of the printing surface substrate because of temperature and humidity considerations and the ink curing process, (7) the printing process due to creep and fatigue that is inherent in the dynamic force loads of the process, etc.
The variation in the ultimate printed image from that of the original art work or desired image has been noted to be substantially linear along the sides of the image. These variations are considered highly significant when printing multiple colors or color sheets, or images that must align to subsequent manufacturing steps, such as die cutting, embossing, drilling, folding, etc., since one sheet may be misaligned with respect to another.